The conventional method of maintaining electroless copper plating solutions in commercial or industrial plants making printed circuit boards and similar workpieces has commonly been by more-or-less frequent manual analysis of the plating bath solution while the plating operation is underway, and then manually making corrective batch additions of components on the basis of the analysis data to replace those consumed. A disadvantage of such system is that by the time the analysis is performed and the replenishment addition requirements are calculated, the plating solution, assuming it has been operating continuously, will have undergone further compositional depletion so that the component levels calculated may be as much as 10% to 20% inaccurate at the time the additions are actually made to the bath.
If the workload in the plating bath is reasonably constant or can be calculated, it is possible to program the additions so that they can be periodically made with some degree of success. However, it is still necessary to verify the concentrations by analysis at least several times during the working day.
In order to eliminate the time lag of manual analysis, and the uncertainty of programmed periodic or batch additions, attempts have been made to automate the analysis and control of the additions on a continuous basis. It is known, of course, that the principle reaction occurring in a plating bath during plating operation is the one represented by the following equation:
Cu.sup.++ +2HCHO+4OH.sup.-.fwdarw. Cu.degree.+H.sub.2 +2H.sub.2 O+2HCOO.sup.- (I)
As will be seen by this equation, the three consumable ingredients, copper, formaldehyde and alkali metal hydroxide, react in a definite stoichiometric ratio and must be replenished in the same ratio to maintain the composition constant. It would appear that, because of this stoichiometric relationship, monitoring any one of these ingredients would provide the necessary information for controlling the replenishment of all three.
In practice, however, there are side reactions which take place independently of the main reaction described above. The most serious of these side reactions is the well-known Canizzaro reaction where formaldehyde and hydroxide interact:
2HCHO+OH.sup.- .fwdarw.CH.sub.3 OH+2HCOO.sup.- (II)
It thus becomes apparent that, in addition to copper, either formaldehyde or hydroxide must also be monitored. By monitoring either of these, the other un-monitored component can be calculated and additions programmed in the required ratio to the addition of the monitored component.
Thus a two-component monitoring control can be established using copper and either hydroxide or formaldehyde, to serve as a basis for programming a control system. U.S. Pat. No. 3,532,519 discloses such a method by monitoring copper and hydroxide. In the method disclosed, a sample stream from the plating bath is pumped through a colorimeter for copper determination, and through a pH meter for a determination of the pH of the bath. The patented system provides that, when a preselected set-point is indicated by either the colorimeter or pH indicator, a relay activates an appropriate pump to introduce aqueous alkali hydroxide solution and/or mixed formaldehyde and copper salt solution, until the sample readings taken from the bath again return to normal or pre-set condition. This method is also summarized in "GALVANOTECHNIK," 61(3), 215(1970) by W. Immel.
These prior methods have been found less than satisfactory with modern highly-active electroless copper solutions, the reasons being that the copper in such solutions will undergo autocatalytic deposition after relatively short periods of operation of the system, producing deposition on the colorimeter cell walls as well as on the pH electrodes, causing inaccurate readings and unreliable functioning of both control systems. Also, the pH of the operating bath is not a reliable indicator of the hydroxide concentration under the conditions employed, since modern plating solutions operate at a pH of 12.5 or higher where the reading is no longer linear with hydroxide concentration due to buffering and sodium ion interference.